1. Field of the Invention
This invention relates to a circuit arrangement for controlling the program timer of a laundry dryer as a function of the moisture-dependent resistance of the laundry, via which a storage capacitor is charged from a d-c voltage source.
2. Description of the Prior Art
In German Published Prosecuted Application No. 17 63 659, a device for controlling drum dryers as a function of the degree of dryness of the laundry is described. The degree of dryness of the laundry is sensed by means of electrodes arranged in the laundry drum. The electrodes are temporarily bridged by the laundry. The resistance between the electrodes have a different value depending on the degree of dryness of the laundry. In this device, the voltage at the storage capacitor, which varies according to the resistance of the laundry, is fed to an integrator consisting of a resistor and a capacitor.
Since the resistance of the laundry becomes high for residual moisture values lower than about 25 to 30%, the charging time constant exceeds the discharging time constant, so that no sufficiently accurate measurement can be obtained in this moisture range without the use of additional means.
The device for the moisture-dependent control for a laundry dryer, described in German Published Prosecuted Application No. 19 35 511, attempts to eliminate this disadvantage. The charging of the capacitor is controlled here by an adjustable pulse generator. During the pulse intervals, the capacitor can discharge through the resistance of the laundry, depending on the duty cycle set and the residual moisture content. The averaged charging voltage is used as the switching criterion for a threshold switch. At residual moisture values lower than about 20%, however, the problem of electrostatic charging of the moving pieces of laundry which transfer their charge to the capacitor from time to time arises with greater frequency. Under these conditions, the charging voltage changes abruptly, which changed voltage can at times simulate a condition that the desired residual moisture has been reached, and thus trigger the threshold switch at the wrong time. An accurate and reliable determination of the desired residual moisture can therefore not be achieved with this method either.
The present invention starts out with a method of drying, in which the thermal energy supplied into the dryer system during the heating phase is used for post-drying after the heater is switched off. During this post-drying phase, a part of the moisture still escapes, thereby saving a substantial amount of energy. In general, the post-drying phase is initiated during the heating phase, when a residual moisture value obtained with the above-described measuring method is reached. Since it is impossible to accurately follow the residual moisture values below about 25% by known methods, the heating phase and the length of the post-drying phase are limited in time and thereby, limit the approach to a desired ultimate residual moisture of the laundry. The smaller the residual moisture values at the end of the heating phase, the more accurately the value of the ultimate residual moisture can be held.